This invention relates to pressure sensitive film composites, and more specifically to such composites having excellent adhesion to plasticized vinyl substrates, both initially and after an extended period of time, and at varying temperatures.
Polyvinyl chloride, or as it is more commonly referred to, "vinyl", is one of the most versatile and widely used synthetic polymers in today's society. For many uses, it is desired that the vinyl be flexible, and thus a plasticizer is compounded with the vinyl. This allows preparation of materials having a myriad of uses.
Such plasticized vinyls typically contain from about 15 to about 50% by weight of monomeric or polymeric plasticizers; because monomeric plasticizers are less expensive, they are commonly employed. Unfortunately, however, monomeric plasticizers are of relatively low molecular weight, thus tending to migrate to the surface of the vinyl film, and into materials with which the vinyl surface comes in contact. This latter tendency is well known; see, for example, U.S. Pat. No. 4,284,681. Therefore, when a pressure sensitive adhesive is applied to a plasticized vinyl substrate and allowed to remain in contact therewith for an extended period of time, plasticizer from the vinyl tends to migrate into the adhesive, softening it and causing a significant decrease in adhesion, see, for example, U.K. Pat No. 1,051,125.
If the vinyl article is, for example, a flexible sign material, a banner, or a truck tarpaulin which is decorated with a graphic marking film attached to the vinyl with a pressure sensitive adhesive, the graphic film will lose adhesion and may become undesirably wrinkled after migration of the plasticizer from the vinyl into the adhesive. In addition, when severe flexing or flapping of the vinyl occurs, such as with canvas-sided trucks, and especially at low temperatures, cracking of the graphic film can result.
Attempts have been made to solve the plasticizer migration problem by interposing an impenetrable barrier layer between the vinyl and the adhesive; see, e.g., U.S. Pat Nos. 4,045,600 and 4,605,592, and E.P. Pat. Spec. 103,407. Others have disclosed adhesives taught to be less susceptible to attack by plasticizer; see e.g. Graziano et al, Development of Acrylic Latex Pressure Sensitive Adhesive for Plasticized PVC Films, Journal of Plastic Film & Sheeting, Vol. 2, April 1986, pp. 95-110. One commercially available tape widely used for adhering to plasticized vinyl is based on an adhesive which is a terpolymer of 2-ethyl hexyl acrylate, vinyl acetate and ethyl acetate.
Yet another attempt to prevent undesired plasticizer migration into an an adhesive has been to incorporate plasticizer itself into the adhesive, thereby reducing the plasticizer gradient between the contacting layers; see, e.g., U.S. Pat. No. 4,272,573; U.K. Pat. Spec. 2,162,191-A; and E.P. Pat. Spec. 150,978.
Despite limited success achieved by the attempts discussed above, there remains a strong commercial desire to provide a graphic film which can be adhered to flexible vinyl articles such as canvas truck sides, awnings, signing, etc., which is subject to extremes of temperature variation as well as severe flexing, and for extended periods of time. Applicants have now discovered that such can be achieved when the adhesive used for attaching the graphic film to such vinyl articles is based on nitrile butadiene rubber (NBR).